The use of free available chlorine as a disinfectant for swimming pool water and other water supplies, continues to be popular. Since the amount of free available chlorine in an aqueous solution relates directly to the disinfecting or sanitizing activity thereof, a test which rapidly and accurately measures free available chlorine has long been desired.
By comparison, chlorine associated with organic chloramines, is not believed to be effective for disinfecting. Accordingly, a test system that measures chloramines in addition to free available chlorine, is not truly indicative of the level of disinfectant.
Colorimetric tests for chlorine solutions using o-tolidine or an azine compound as a colorimetric indicator, are known. Illustrative are U.S. Pat. Nos. 1,986,403 to Lehmkuhl and 4,092,115 to Rupe. For determining free available chlorine, Rupe describes the use of an azine compound, a buffer system having a pH of from about 3.5 to 8.5, preferably about 6.0, and the use of phosphate buffer. A drawback is that the azine reagents do not produce a stable colored reaction product. Also known as exemplified by Canadian Patent No. 818,344 to Rupe, is a chloride test paper containing a diaminodiphenyl compound, preferably o-tolidine, and a cupric salt.
Also known as illustrated by U.S. Pat. No. 4,339,243 to Magers et al, is the stabilization of benzidine-type indicators with various enhancers. Evaluated therein is a test paper containing 3,3',5,5'-tetramethylbenzidine (TMB) for sodium hypochlorite solution, and prepared using pH 5 buffer. Various carrier matrices including a matrix of polyamide fibers, are disclosed.
As exemplified by U.S. Pat. Nos. 4,273,868 to Walter, 4,385,114 Guthlein et al and 4,390,621 to Bauer, also known is a test paper containing TMB for the detection of hydrogen peroxide, glucose, cholesterol and so forth. Coagents include a peroxide, oxidase or other enzymatic coagent. As illustrated in U.S. Pat. Nos. 4,273,868 and 4,385,114, multi-step impregnation by a free base formulation involves successive dips in a water-containing buffer solution, and an organic liquid-containing solution of TMB. As illustrated in the Walter patent, an interpolymer of methylvinyl ether and maleic anhydride may be used for color enhancement. As exemplified in the Bauer patent, a polyethoxylated fatty alcohol, nonionic surfactant may be used for color uniformity.
Test strips typically include an absorptive carrier bearing the colorimetric indicator, for instance, absorptive paper impregnated with the indicator, or a support having the absorbent carrier fixed thereto. A modified reagent strip including a porous matrix bearing the indicator and having exposed opposite faces, is illustrated by U.S. Pat. No. 5,187,100. A blood sample is placed in an aperture that exposes a face of the matrix, and the blood seeps from capillary effect to the opposite face. In a similar strip, the matrix is disposed between support members provided with apertures generally in line with each other.
Also known as exemplified by U.S. Pat. No. 4,092,115 to Rupe, earlier mentioned, is a test strip including a wick member enclosed in a fluid impervious sheath, and including an aperture exposing a portion of the wick member bearing colorimetric indicator. A drawback of this type of device is non-uniform color development. Also, as shown by U.S. Pat. No. 3,510,263, the faces of a chromatographic test strip may be coated with a water repellent material, and the edges may be uncoated.
Areas of use for colorimetric test strips include tap water quality testing, industrial and environmental testing, pool and spa testing, lake and stream testing, aquarium testing and other types of water testing. There continues to be a need for a colorimetric test strip of enhanced sensitivity for determination of free chlorine. It is essential that a test strip have the necessary detection capability. Stability of the indicator, uniform color development and stability of the developed color would be advantageous. Moreover, a broad range of sensitivity would be beneficial. It would be also advantageous for the test strip to be economical to manufacture.